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Chapter 2

Chapter 2. Point-to-Point Protocols and Links. Section 2.1. Introduction. 2.1 Introduction. Physical communication links Data link control i.e. point-to-point protocols Physical links : requires background in Linear system theory Random process Modern communication theory Recall

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Chapter 2

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  1. Chapter 2 Point-to-Point Protocols and Links

  2. Section 2.1 Introduction

  3. 2.1 Introduction • Physical communication links • Data link control • i.e. point-to-point protocols • Physical links : requires background in • Linear system theory • Random process • Modern communication theory • Recall • Chapter 1 section 1.3.1 page 34

  4. Introduction

  5. Introduction • Our major problem in DLC : correct bit error • Error detection & correction • ARQ ( Automatic Repeat request) • Header + packet + trailer => frame

  6. Section 2.2 Physical layer Channels & modems

  7. 2.2 Physical layer:channels & modems • Skip,will be discussed if necessary

  8. Section 2.3 Error Detection

  9. 2.3 Error Detection • DLC layer is to provide error-free packets to next layer up

  10. 2.3.1 Single Parity checks • Parity checks bit is the sum, modulo 2 , of the bits in the original bit string • Total number of 1`s in an encoded string is always even • Detect single bit error only.And , odd number of bit errors

  11. 2.3.2 Horizontal & Vertical Parity Checks

  12. Horizontal & Vertical Parity Checks • Common use for ASCII encoded characters • Cannot detect four errors confined to 2 rows and 2 columns

  13. 2.3.3 Parity Check Codes

  14. Parity Check Codes • Effectiveness of a code for error detection • Minimum distance of the code • Single parity check is 2 • Horizontal & Vertical is 4 • Burst-detecting capability • Single parity is 1 • Horizontal & Vertical is 1+length of row • Probability that a completely random string will be accepted as error-free

  15. Parity Check Codes • E.g. • Minimum distance • Single parity check is 2 • Horizontal & Vertical is 4 • Burst-detecting • Single parity check is 1 • Horizontal & Vertical is 1 + length of row

  16. Parity Check Codes

  17. Figure 2.15

  18. 2.3.4 Cyclic Redundancy Checks(CRC)

  19. Cyclic Redundancy Checks(CRC)

  20. Cyclic Redundancy Checks(CRC) L=3 S(D)=D2+1

  21. Cyclic Redundancy Checks(CRC)

  22. Cyclic Redundancy Checks(CRC)

  23. Cyclic Redundancy Checks(CRC)

  24. Cyclic Redundancy Checks(CRC)

  25. Cyclic Redundancy Checks(CRC)

  26. Cyclic Redundancy Checks(CRC)

  27. Cyclic Redundancy Checks(CRC)

  28. Cyclic Redundancy Checks(CRC)

  29. Section 2.4 ARQ: Retransmission Strategies

  30. 2.4 ARQ:Retransmission Strategies • 2 aspects of retx algorithms or protocols • Succeed in releasing each packet,one oad only once without errors • Efficiency releasing unnecessary waiting & unnecessary retx

  31. ARQ:Retransmission Strategies • We assume , all frames containing transmission errors are detected • Delay is arbitrary • Frame may be “lost” & never arrive • Frames arrive in the same order as transmitted

  32. ARQ:Retransmission Strategies

  33. 2.4.1 Stop-and-wait ARQ • Each packet has been received correctly before initiating tx of next packet • If • Error free • Acknowledge , Ack • Error frame • Negative acknowledgement , NAK • Ack & NAK is protect with a CRC • Ack lost or NAK • Resend the old packet

  34. Stop-and-wait ARQ

  35. Stop-and-wait ARQ

  36. Stop-and-wait ARQ • Avoid this problem,returns the number of next packet awaited • Piggyback

  37. Stop-and-wait ARQ

  38. Fig 2.21

  39. Stop-and-wait ARQ • The algorithm for A to B • At node A • SN0 • Assign SN to the new packet • Tx SN-th frame • If receive from B with B RN>SN,SNRN,go to Step2 If no received frame from B,timeout , go to Step 3

  40. Stop-and-wait ARQ • Continued • At node B • RN0,repeat step2 &3 forever • If error-free frame received with SN==RN,RN++ • Within bounded delay after receiving error-free frame send a frame to A containing RN

  41. Stop-and-wait ARQ • Correctness of stop-and-wait • An algorithm is safe if it never produces an incorrect result • An algorithm is live if it can continue forever to produce results • Safety • Initially, node B awaiting packet 0 , and only packet 0 is released.Subsequently,node B has released all packets in order,up to , but not including ,packet RN

  42. Stop-and-wait ARQ

  43. Stop-and-wait ARQ

  44. Stop-and-wait ARQ

  45. Stop-and-wait ARQ

  46. Stop-and-wait ARQ • One trouble with Stop-and-wait • SN become arbitrarily large with increasing time • Given our assumption that frames travel in order on the link,SN modulus 2 is sufficient!

  47. Stop-and-wait ARQ

  48. 2.4.2 Go Back n ARQ • Several successive packets can be sent without waiting for the next packet to be requested • Accept packets only in the correct order , and send RN back • RN is to acknowledge all packets prior to RN and to request packet RN

  49. Go Back n ARQ

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